Conventionally, a compressor rotor stage in a gas turbine engine comprises a plurality of radially extending blades mounted on a disc. The blades are mounted on the disc by inserting a root portion of the blade in a complementary retention groove in the outer face of the disc periphery. To ensure a smooth radially inner surface for air to flow over as it passes through the stage, annulus fillers can be used to bridge the spaces between adjacent blades. Typically, a seal between the annulus fillers and the adjacent fan blades is also provided by resilient strips bonded to the annulus fillers adjacent the fan blades.
Annulus fillers of this type are commonly used in the fan stage of gas turbine engines. The fillers may be manufactured from relatively lightweight materials and, in the event of damage, may be replaced independently of the blades.
It is known to provide annulus fillers with features for removably attaching them to the rotor disc. An annulus filler may be provided with axially spaced hook members, the hook members sliding into engagement with respective parts of the rotor disc and/or a component located axially behind the rotor assembly, for example a rear fan air seal. FIG. 1 shows an example of such an annulus filler viewed from the side. In use, the upper surface or lid 2 of the annulus filler 1 bridges the gap between two adjacent fan blades (not shown) and defines the inner wall of the flow annulus of a fan stage. The annulus filler 1 is mounted on a fan disc (not shown) by two hook members 3 and 4, respectively towards the forward and rearward ends of the annulus filler. It is also attached to a front support ring (not shown) at axial retention flange 5, the support ring itself attaching to the front of the fan disc. The two opposed side faces of the annulus filler are provided with respective seal strips (not shown) and confront the aerofoil surfaces of the adjacent fan blades. Typically the annulus filler is a machined aluminium alloy forging.
Retention flange 5 carries an interference fit pin 6 which inserts into a corresponding hole formed in the front support ring. Flange 5, abutting the support ring, resists motion of the annulus filler in the forward axial direction, while pin 6 helps to ensure that the annulus filler occupies the correct angular position on the disc.
Annulus fillers of this type are self-loading in that, as a rotating component, the majority of forces on the filler are generated by its own mass. However, under birdstrike or blade-off conditions, a blade can deflect at the annulus filler position and apply a pushing force to the filler in a lateral, circumferential direction. The hook contact faces of the annulus filler shown in FIG. 1 and of the fan disc to which, in use, it is mounted are curved around a radius of the disc so that under the force of a deflecting blade the filler will tend to rotate at the hook members around the centre line of the engine.
However, as the axial retention flange 5 is pinned, and sometimes also bolted, to the front support ring, this rotation leads to twisting of the annulus filler. The twisting can cause damage or failure of the filler, typically at the lid 2 between the forward hook member and the retention flange.